1. Field of the Invention
The invention relates to a compact torque and index detection apparatus that combines the functions of a torque sensor and an index sensor.
2. Description of Related Art
Electric power steering devices for a vehicle include a steering wheel with which the driver performs a steering operation, an input shaft rotated by the steering wheel, a connecting shaft (torsion bar) to which torque (steering torque) is applied by rotation of the input shaft, an output shaft rotated by the torque applied to the connecting shaft, a torque sensor that senses torque applied to the connecting shaft, and an auxiliary motor that is driven in accordance with the steering torque sensed by the torque sensor. The driving force of the auxiliary motor is transmitted to a steering structure connected to the output shaft to thereby assist the driver's steering operation.
In electric power steering devices, in addition to steering torque, the orientation of steered wheels (steering angle) that changes with the steering operation also may be sensed by a steering angle sensor.
An example of a technique that integrates a torque sensor and a steering angle sensor is described in Japanese Patent Application Publication No. 2007-269281. With regard to the torque sensor, JP-A-2007-269281 describes that the torque sensor includes a cylindrical magnet formed by a multipolar magnet provided on the input shaft and having N-poles and S-poles arranged alternately side by side in the circumferential direction, a pair of yoke rings made of a magnetic material which are provided on the output shaft and placed within the magnetic field of the cylindrical magnet to form a magnetic circuit, a pair of magnetic collecting rings made of a magnetic material which are magnetically coupled to the yoke rings and guide magnetic flux from the yoke rings to thereby collect the magnetic flux, and a magnetic sensing element (magnetic sensor) that senses the magnetic flux guided by the magnetic collecting rings, and that the torque sensor senses torque applied to the connecting shaft on the basis of the output of the magnetic sensing element. Also, with regard to the steering angle sensor, JP-A-2007-269281 describes that the steering angle sensor includes a pair of gears with different numbers of teeth which rotate in synchronization with rotation of the output shaft around two axes located in parallel to the output shaft, and a plurality of magnetic sensing elements that sense the rotation angles of the respective gears, and that the steering angle sensor senses steering angle on the basis of the combination of the rotation angles sensed by the respective magnetic sensing elements. The steering angle sensor as described in JP-A-2007-269281 is referred to as mechanical steering angle sensor.
Other than the mechanical steering angle sensor as described in JP-A-2007-269281, another type of steering angle sensor is an index-type steering angle sensor which sets a reference position for the rotational position of the output shaft, senses the number of the rotation of the output shaft by sensing (indexing) the presence of the output shaft in the reference rotational position, and uses the sensed rotational speed for sensing steering angle. For example, electric power steering devices are equipped with an auxiliary motor, and the relative angle of the output shaft can be obtained on the basis of the rotation angle of the auxiliary motor. In the case of using an index-type steering angle sensor, steering angle can be sensed by obtaining the absolute angle of the output shaft on the basis of this relative angle of the output shaft obtained from the auxiliary motor, and the number of the rotation of the output shaft sensed by sensing the presence of the output shaft in the reference rotational position.
By directing attention to the technique of an index-type steering angle sensor, the present inventors have conceived of installing a torque sensor and an index sensor serving as an index-type steering angle sensor together in an electric power steering device.
However, merely installing a torque sensor and an index sensor together means that two sensors are merely installed together, which not only increases the number of parts but also detracts from compactness.